This invention relates generally to random refresh (as opposed to raster) display systems. More specifically, this invention relates to random refresh display systems that generate deflection signals from digital commands specifying vectors to be drawn on the display. Such display systems have application in architectural and engineering drawing, seismic waveform display, kinematics, simulation, and other display situations. Typically, an image being displayed comprises many discrete lines (vectors), interconnected in a variety of ways. Thus a set of vectors defines the image.
Cathode-ray tube (CRT) display monitors are well known for the graphic display of electrically computed or otherwise processed information. Basically, in a cathode-ray tube, a stream of electrons is directed from an electron gun past magnetic deflection coils or pairs of electrostatic deflection plates (depending upon whether the tube is a magnetic deflection or electrostatic deflection tube) towards a phosphorized screen. The point at which the electron beam formed by the stream of electrons impinges the screen is temporarily (except in applications wherein storage tubes are used) illuminated. Two pairs of deflection plates control the position of the resulting illuminated spot on the screen. The deflection of the illuminated spot from the center of the screen depends upon the magnitude of the current or voltage applied across the deflection coils or plates. One deflection coil or pair of deflection plates controls the deflection of the electron beam vertically in the Y-direction, and the other pair of plates controls the deflection horizontally in the X-direction. Simultaneously, the coils or pairs of plates can direct the electron beam to impinge on any point within the range of the screen, and predetermined voltage levels across the horizontal and vertical deflection coils or plates correspond to definite X- and Y-coordinate positions of the illuminated spot on the screen. The graphic display control system set forth in this patent generates the deflection signals (known in the art as the "X" and "Y" signals) and beam "on" or "off" signals (known in the art as the "video" signal) for driving such a display. In addition it generates a signal for controlling the brightness of each vector drawn (known in the art as the "Z" signal).
An image, defined by one or more vectors, persists on the face of the CRT for only a short period of time and is therefore "refreshed", usually at a rate of about 60 times/second, by redrawing the vectors defining the displayed image. An insufficiently frequent refresh results in apparent "flicker" of the displayed image. Also the brightness of a line is a function of its drawing speed. The faster a vector is drawn, the less bright it will appear. Thus a vector drawn at other than a constant speed will appear to have brightness changes along its length.
Generally, a digital data source, commonly computer-based although not necessarily so, provides vector information defining pictures to be drawn in the form of vector and display commands. These commands are processed by either a digital or an analog vector generator into the "X" and "Y" signals that are applied to the "X" and "Y" position inputs of the display monitor. These X and Y positional inputs couple the "X" and "Y" signals to display amplifiers which drive either a yoke containing magnetic deflection coils or pairs of electrostatic deflection plates to cause the displacement of a writing beam, from the cathode of the CRT, impinging upon the face of the tube. The present invention represents a novel technique for processing the digital commands from a digital data source into the "X" and "Y" positional signals brightness ("Z") signal and video signal suitable for driving a conventional display monitor.
The primary objective of all random refresh display systems, in general, is to display an accurate image representation of the vectors defined by the commands coupled thereto and to make the display as eye-pleasing as possible. More specifically, such systems attempt to display many picture lines with clearly defined and accurate beginning and end points, sharp corners, without screen "flicker", and with adjustment-free operation. These ideals have been difficult, if not impossible, to attain.
The prior art has developed along two separate and distinct lines. Systems of one such line of development utilize analog vector generators for generating the X and Y positional signals. Systems of the other line utilize digital vector generators to generate the X and Y positional signals.
The classic analog vector generator-type system uses an integrator to generate a "ramp" signal defining a smooth position change from a first to a second position; i.e. a first X coordinate to a second X coordinate and a first Y coordinate to a second Y coordinate.
An example of the classic integrator type vector generator is shown in U.S. Pat. No. 3,800,183-Halio et al (1974). There is an inherent inaccuracy in generating position signals with an integrator because small differences between the X and Y circuits cause a cumulative error in position and the drawn line does not pass through the desired end point. In fact, the longer the vector being drawn, the more pronounced this error becomes. Systems were devised for correcting this error. In such correcting systems difference between the actual position of the "dot" on a display tube and that called for by the vector command being executed is sensed. A correction signal is generated to bring the actual dot position into alignment with the called for position. In U.S. Pat. No. 3,800,183, a comparator circuit is used to sense the difference between actual and called for positions. Other systems, such as shown in U.S. Pat. No. 4,032,768-Rieger (1977) utilize complex feedback arrangements for correcting the cumulative error of analog type vector generators. Unfortunately, with such correcting systems complicated alignments are required in order to cause the X and Y deflections to reach their final destination at the same time.
Digital vector generators, on the other hand, are substantially adjustment free but may require expensive logic for implementation. One such example of a digital vector generator is shown in U.S. Pat. No. 3,723,802-Granberg et al (1973). Digital vector generators compute every point through which the beam must pass and therefore generally draw slower than analog vector generators. In addition, digital vector generators do not generate as smooth a picture line. Rather, the picture line drawn on the face of a monitor by digital vector generators is a staircase resulting from the stepwise jumps of X and Y position.
In both analog and digital vector generator systems, there has always been a tradeoff between the amount of picture information that can be accurately displayed and the flicker associated with the display by virtue of the continuous refreshing of the image formed on the display. As drawing speed is increased in order to minimize flicker by refreshing the displayed image more often, inaccuracies in the displayed image are accentuated, corners become less sharp and edges may not properly meet.